1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which stores “1”- and “0”-information using a tunneling magnetoresistive effect.
2. Description of the Related Art
In recent years, many memories which store information by new principles have been proposed. One of them is a memory using the tunneling magnetoresistive (to be referred to as TMR hereinafter) effect proposed by Roy Scheuerlein et al. (e.g., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC2000 Technical Digest, p. 128).
A magnetic random access memory stores “1”- and “0”-information using MTJ (Magnetic Tunnel Junction) elements. An MTJ element has a structure in which an insulating layer (tunneling barrier) is sandwiched between two magnetic layers (ferromagnetic layers), as shown in FIG. 109. Information to be stored in the MTJ element is determined on the basis of whether the magnetizing directions of the two magnetic layers are parallel or antiparallel.
As shown in FIG. 110, “parallel” means that the two magnetic layers have the same magnetizing direction (magnetizing direction). “Antiparallel” means that the two magnetic layers have opposite magnetizing directions (the arrows indicate the magnetizing directions).
Normally, an antiferromagnetic layer is arranged on the side of one of the two magnetic layers. The antiferromagnetic layer serves as a member which fixes the magnetizing direction of one magnetic layer and changes only the magnetizing direction of the other magnetic layer, thereby easily rewriting information.
The magnetic layer whose magnetizing direction is fixed is called a fixed layer or pinning layer. The magnetic layer whose magnetizing direction can freely be changed is called a free layer or storing layer.
As shown in FIG. 110, when the magnetizing directions of the two magnetic layers are parallel, the tunneling resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers is minimized. This state is a “1”-state. When the magnetizing directions of the two magnetic layers are antiparallel, the tunneling resistance of the insulating layer (tunneling barrier) sandwiched between the two magnetic layers of the MTJ element is maximized. This state is a “0”-state.
The write operation principle for an MTJ element will be briefly described next with reference to FIG. 111.
MTJ elements are arranged at the intersections between write word lines and data lines (read/write bit lines) which cross each other. A write is done by supplying a current to each of a write word line and a data line and setting the magnetizing direction of an MTJ element in the parallel or antiparallel state using a magnetic field generated by the currents flowing through the two lines.
For example, assume that the easy-axis (axis of easy magnetization or easy magnetization axis) of an MTJ element corresponds to the X-direction, a write word line runs in the X-direction, and a data line runs in the Y-direction perpendicular to the X-direction. In the write mode, a current that flows in one direction is supplied to the write word line, and a current that flows in one or the other direction is supplied to the data line in accordance with write data.
When a current that flows in one direction is supplied to the data line, the magnetizing direction of the MTJ element is set in the parallel state (“1”-state). On the other hand, when a current that flows in the other direction is supplied to the data line, the magnetizing direction of the MTJ element is set in the antiparallel state (“0”-state).
The magnetizing direction of the MTJ element changes in accordance with the following mechanism.
As is indicated by the TMR curve in FIG. 112, when a magnetic field Hy is applied in the longitudinal (easy-axis) direction of an MTJ element, the resistance value of the MTJ element changes by, e.g., about 17%. The change ratio, i.e., the ratio of the resistance difference between the anti-parallel state and the parallel state and the resistance of the parallel state is called “MR ratio”.
Note that the MR ratio changes depending on the structure, composition and morphology of the MTJ element. Currently, even an MTJ element with an MR ratio of about 50% is available.
The synthesized magnetic field of the magnetic field Hy in the easy-axis direction and a magnetic field Hx in the hard-axis (axis of hard magnetization or hard magnetization axis) direction is applied to the MTJ element. As indicated by the solid line in FIG. 113, the intensity of the magnetic field Hy in the easy-axis direction, which is necessary for changing the resistance value of the MTJ element, changes depending on the intensity of the magnetic field Hx in the hard-axis direction. When this phenomenon is used, data can be written in only an MTJ element that is present at the intersection between a selected write word line and a selected data line in memory cells arranged in an array.
This mechanism will be described in more detail using the asteroid curve shown in FIG. 113.
An MTJ element has an asteroid curve indicated by, e.g., the solid line in FIG. 113. More specifically, when the intensity of the synthesized magnetic field of the magnetic field Hy in the easy-axis direction and the magnetic field Hx in the hard-axis direction is outside (e.g., at the position indicated by the filled circle) the asteroid curve (solid line), the magnetizing direction of the magnetic layer can be reversed.
To the contrast, when the intensity of the synthesized magnetic field of the magnetic field Hy in the easy-axis direction and the magnetic field Hx in the hard-axis direction is inside (e.g., at the position indicated by the open circle) the asteroid curve (solid line), the magnetizing direction of the magnetic layer cannot be reversed.
Hence, when the intensity of the magnetic field Hy in the easy-axis direction and that of the magnetic field Hx in the hard-axis direction are changed to change the position of the intensity of the synthesized magnetic field in the Hx-Hy plane, the data write for the MTJ element can be controlled.
A read can easily be performed by supplying a current to a selected MTJ element and detecting the resistance value of the MTJ element.
For example, switch elements are connected in series to the MTJ elements. Only the switch element connected to a selected read word line is turned on to form a current path. Consequently, a current flows to only the selected MTJ element. Hence, data of the MTJ element can be read.
In the magnetic random access memory, as described above, the data write is executed by, e.g., supplying write currents to a write word line and a data line (read/write bit line) and causing a thus generated synthesized magnetic field to act on an MTJ element.
In the write operation, it is necessary to always accurately write data in an MTJ element. That is, a stable write characteristics is necessary. Stabilizing the write characteristics is especially important when data (the state of an MTJ element) stored in an MTJ element and write data are different. In such a case, the magnetized state (magnetizing direction) of the storing layer of the MTJ element must be stably inverted.
Conventionally, as a write method invented from the viewpoint of stabilizing the write characteristics, a method described in, e.g., U.S. Pat. No. 6,081,445 “Method to Write/Read MRAM Arrays” is known.
In this method, as shown in FIG. 114, first, the magnetic field Hx in the hard-axis direction is caused to act on the MTJ element to align the magnetizing direction at the end portion of the storing layer of the TMR layer to the hard-axis direction ({circle around (1)}). Then, the magnetic field Hy in the easy-axis direction is caused to act on the MTJ element ({circle around (2)}).
In this method, after a write current flows to the write word line, a write current having a direction corresponding to write data flows to the write bit line. The easy-axis of the MTJ element is set along the direction in which the write word line runs.
As described above, the magnetic field Hx extending in the hard-axis direction is made to act on the MTJ element before the magnetic field Hy in the easy-axis direction acts on the MTJ element. As a result, the magnetizing direction at the end portion of the storing layer of the MTJ element is aligned to the hard-axis direction (the magnetizing direction is made unstable). This alignment is performed to prevent changes in the direction of magnetization in the end portion of the storing layer of the MTJ element every time data is written. Hence, the data written is not influenced by the data that has been written before. This helps to enhance the reliability of data-writing.
The inversion of magnetization of the storing layer of the MTJ element starts from the end portion of the storing layer, as shown in FIG. 115. Hence, the intensity of the synthesized magnetic required and the time of applying the magnetic field may change every time data is written, unless the end portion of the storing layer is magnetized in the same direction every time the data-writing starts. The magnetic field Hx extending in the hard-axis direction is made to act on the MTJ element before the magnetic field Hy in the easy-axis direction acts on the MTJ element. In this method, the field Hx is more intense than a magnetic field that should be applied to change the direction of magnetization in the main portion of the storing layer to the hard-axis direction. Thus, the intensity of the field Hx remains unchanged, not influenced by the data that is being written. That is, the field Hx is intense enough to change the direction of magnetization in the end portion of the storing layer, which is more changeable than the direction of magnetization in the main portion. This data-writing method is advantageous in terms of the reproducibility of data, because the magnetic field Hx, which is not influenced by the data being written, can alone determine the direction of magnetization in the end portion.
U.S. Pat. No. 6,081,445 discloses only that the magnetic field Hx in the hard-axis direction is caused to act on the MTJ element, and then, the magnetic field Hy in the easy-axis direction is caused to act on the MTJ element. In this case, it may be impossible to sufficiently invert the magnetization of the storing layer of the MTJ element. In addition, the magnetizing direction at the end portion of the storing layer of the MTJ element is kept aligned to the hard-axis direction even after the write operation.